1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus, and more particularly, to a sheet processing apparatus and an image forming apparatus improving, when a tab sheet having a tab serving as a projection is overlaid on a sheet in a sheet conveyance path to form a sheet bundle to be conveyed and output, an alignment grade of the output sheets including the tab sheet.
2. Description of the Related Art
Conventionally, a sheet processing apparatus that performs various types of processing for a sheet discharged from a main body of an electrophotographic image forming apparatus for forming an image using toner has been mounted as an option. Such sheet processing apparatuses have been contrived to avoid deterioration in productivity occurring when sheet processing, which requires a relatively long processing time, for example, processing for binding sheets into a sheet bundle is performed.
For example, a sheet conveyed from the image forming apparatus main body while a preceding sheet bundle is processed in a processing tray is made to temporarily wait in a conveyance path in the sheet processing apparatus.
A sheet is repeatedly stopped and conveyed with precise timing to overlap a predetermined number of sheets corresponding to a sheet processing time of the preceding sheet bundle to form a sheet bundle in the conveyance path, and the first several sheets included in the subsequent sheet bundle are made to wait, to ensure the sheet processing time of the preceding sheet bundle. After the preceding sheet bundle is discharged from the processing tray, the sheets that are waiting are conveyed onto the processing tray, and are aligned in the sheet conveyance direction by their edges in the sheet conveyance direction abutting on a stopper.
The sheet processing apparatus includes a transporting member that abuts, when the overlapped sheets are conveyed onto the processing tray, on only the uppermost sheet so as to cause edges of the sheets, in the sheet conveyance direction, conveyed onto the processing tray to collide against the stopper.
The lower sheets, on which the transporting member does not abut, collides against the stopper using a frictional force between the sheets generated when the uppermost sheet is transported. At this time, when the uppermost sheet abuts on the stopper earlier than the lower sheets, the lower sheets may be stopped before abutting on the stopper, resulting in alignment failure.
In order to prevent this, when the sheets are overlapped, the sheets can be conveyed onto the processing tray after their edges in the sheet conveyance direction are lined up and while the state is maintained so that the overlapped sheets simultaneously abut on the stopper. When a plurality of sheets is overlapped in the conveyance path, however, their edges in the sheet conveyance direction are difficult to line up due to errors in the length of sheets in the conveyance direction and conveyance errors occurring when conveyance means is driven.
Therefore, sheets are overlapped by previously shifting the lower sheet toward a stopper by a predetermined amount from the upper sheet so that the lower sheet abuts on the stopper earlier than the upper sheet even when the above-mentioned error occurs as discussed in Japanese Patent Application Laid-Open No. 10-194582.
When the sheets abut on the stopper using a frictional force between the sheets, as described above, however, alignment failure may occur depending on the type of sheet to be overlapped, such as a tab sheet, coated paper, or a Z-folded sheet. When a standard sheet is overlaid on a tab sheet having a tab (an index portion, a heading, an index) that projects in the sheet conveyance direction at a position, which differs depending on the individual tab sheet, in a width direction perpendicular to the sheet conveyance direction, for example, alignment failure may occur on the processing tray.
Generally, a tab sheet having a tab serving as an index portion of a sheet bundle including a plurality of sheets is made of thick paper having a larger thickness than that of a normal sheet having no tab, and thus has a greater weight than that of the normal sheet.
When the tab sheet is placed as the second sheet of three sheets, for example, the tab sheet may be unable to transport because the weight thereof is large even if it attempts to abut on the stopper by a transporting force generated by friction with the uppermost sheet. When the tab sheet is the lowermost sheet, a similar phenomenon may also occur.
When coated paper, the surface of which has been subjected to surface processing or coating processing to obtain a surface property suitable for a color image; or a Z-folded sheet obtained by folding a large-format sheet in a Z shape is overlapped on a position other than the uppermost sheet, a similar phenomenon may also occur.
The surface of the coated paper is smoothed by pressure-welding the surface using a metal roller or the like, or applying a special chemical to the surface. When the surface of the coated paper is pressure-welded, the density of the coated paper becomes higher than that of a normal sheet having the same thickness, so that the coated paper has a greater weight than that of the normal sheet. Since the surface of the coated paper is smooth, a frictional force generated between the sheets is small.
A Z-folded sheet can be obtained by folding an A3 size sheet in a Z shape to have an A4 size, for example. The Z-folded sheet, together with a normal sheet of an A4 size, can be bookbound. If the size of the Z-folded sheet after the folding is the same as that of the normal sheet, then the weight of the Z-folded sheet will be larger than that of the normal sheet. Only an edge of the z-folded sheet, which contacts the upper sheet, may be transferred by a frictional force from the upper sheet, and an edge of the z-folded sheet, which does not contact the upper sheet, may remain un-moved.